1. Field of the Invention
This invention relates to a process and means for the purification of cellulose ethers and derivatives thereof.
2. Statement of Related Art
Cellulose ethers are industrially produced by the reaction of cellulose in the form of powder, short-staple fibers or chips or sheets with one or more etherifying agents in the presence of sodium hydroxide. Monochloroacetic acid or rather the sodium salt of monochloroacetic acid, methyl chloride, ethylene oxide and/or propylene oxide are often used as the etherifying agents. Depending on the etherification process and the degrees of substitution, the products obtained either have no flocculation point (for example carboxymethyl, hydroxyethyl celluloses) in the usual measuring range from 20.degree. to 100.degree. C. and even higher or become insoluble in the event of an increase in temperature and flocculate (methyl, hydroxypropyl celluloses).
Cellulose ethers with no flocculation points are freed from the usual secondary products, such as sodium chloride glycols, polyglycols, glycolates, by washing with alcohol/water mixtures.
Cellulose ethers having low flocculation points may be freed very easily from the secondary products (sodium chloride, methanol, glycols) by washing with hot water and may be washed free from salts.
The working up of derivatives having flocculation points above 80.degree. to 85.degree. C. presents difficulties. Since there is no single temperature at which all the macromolecules of a solution become insoluble at the same time, it would be better to speak in terms of a flocculation range which is characterized by indication of the flocculation point.
For products such as these, washing with hot water is no longer possible without significant losses of product and clogging of the apparatus used for working up.
The purification of cellulose ethers having relatively high flocculation points and those showing thermoplastic properties presents considerable difficulties because the conventional centrifuges which, basically, would be suitable for the separation and purification of such substances do not permit shear-free discharge of the product, residual layers remaining in the centrifuges generally cannot be avoided without special purification processes and considerable losses of product have to be accepted during centrifuging on account of the relatively coarse sieves used.
The object of the present invention is to provide a purification process for cellulose ethers which may be universally applied to all cellulose ethers. Another object of the invention is a process for, purifying cellulose ether derivatives having high flocculation points in the range from 80.degree. to about 95.degree. C. and higher, and to thermoplastic, more especially propoxylated, products. It is a further object of the invention to provide a process for with minimal product losses which provides shear-free product discharge with no residual layers.